In general, the luminance of an organic electroluminescence (EL) element is dependent upon the drive current supplied to the element, and the luminance of the element increases in proportion to the drive current. Therefore, the power consumption of a display made up of organic EL elements is determined by the average of display luminance. Specifically, unlike liquid crystal displays, the power consumption of organic EL displays varies significantly depending on the displayed image. For example, in an organic EL display, the highest power consumption is required when displaying an all-white image, whereas, in the case of a typical natural image, power consumption which is approximately 20 to 40% that for all-white is considered to be sufficient.
However, because power source circuit design and battery capacity entail designing which assumes the case where the power consumption of a display becomes its highest, it is necessary to consider power consumption that is 3 to 4 times that for the typical natural image, thus becoming a hindrance to the lowering of power consumption and the miniaturization of devices.
In response, there is conventionally proposed a technique which reduces power consumption with practically no drop in display luminance, by detecting the peak value of video data and regulating the cathode voltage of the organic EL elements based on such detected data so as to reduce the power source voltage (for example, see Patent Literature (PTL) 1).
However, especially, in the case of the organic EL displays, only the above-stated regulation of the power source voltage based on video data is insufficient from the perspective of reducing power consumption. Since an organic EL element is a current-driven element, a current flows through anode-side power wires and cathode-side power wires, and a voltage drop proportionate to wire resistance occurs. When a measure is taken in consideration of this voltage drop, the reduction in power consumption is achieved. The measure for the above-stated voltage drop is described.
FIG. 20 is a circuit diagram illustrating a circuit configuration of a pixel which drives an organic EL element proposed in PTL 2.
In the pixel circuit configuration disclosed in the PTL 2, in the case where a source-drain voltage of a driver transistor Q1 that flows a current in and thereby drives an organic EL element is high and its operating point is in a saturation region even when a voltage drop occurs in the power wire, it is possible to appropriately display images set based on a data line voltage according to video signals.
However, in the case where the source-drain voltage of the driver transistor Q1 is low and its operating point is in a linear region, resistance components of an organic EL element OLED and a switch transistor Q4 and the source-drain voltage of the driver transistor Q1 are largely influenced, causing a failure to appropriately display images.
As such, in order that the operating point of the driver transistor Q1 is in the saturation region, a voltage drop margin for compensating for a voltage drop is added when setting the power source voltage to be supplied to the display.
In the same manner as the previously described power source circuit design and battery capacity, since the voltage drop margin for compensating for a voltage drop is set assuming the case where the voltage drop amount of the display becomes highest, unnecessary power is consumed for typical natural images.
In a small-sized display intended for mobile device use, the panel current is small and thus, compared to the voltage to be consumed by pixels, the voltage drop margin for compensating for a voltage drop is negligibly small.
However, when the current increases with the enlargement of panels, the voltage drop occurring in the power wire no longer becomes negligible.
Meanwhile, PTL 3 discloses a technique for an electronic display including a current-driven light-emitting unit, in which a voltage drop amount on a feeder wire is calculated from wire resistance of a power supply line and a pixel current and then from the voltage drop amount, the minimum required power source voltage is calculated, to regulate a power source voltage. Furthermore, the PTL 3 discloses a technique of coupling the calculated voltage drop amount to image signals provided from outside, thereby generating a voltage for determining luminance of the light-emitting unit, which is written into a capacitor. Through these techniques, the power consumption can be reduced and it becomes possible to reduce luminance variations in the electronic display disclosed in the PTL 3.